1. Field of the Invention
The present invention relates to a process for the production of expandable granules of thermoplastic polymers and to the relative apparatus.
More particularly, the present invention relates to a process for the production of extrusion of non-oriented and non-stressed expandable polymers, particularly suitable for the production of expanded articles.
2. Description of the Prior Art
It is known that the thermoplastic polymers, such as for instance polystyrene, may be expanded by incorporating intimately into the polymer an expanding agent and then by heating the polymer under such conditions that its viscosity lowers and the expanding agent is allowed to vaporize and to form quite a number of closed spaces or cells in the polymer.
When polystyrene is concerned, as a rule, an organic liquid vaporizable substance, such as for instance a hydrocarbon containing from 4 to 6 carbon atoms is used as expanding agent.
The amount of expanding agent contained in the polymer may range between 2 and 10% by weight.
The expandable polymers are generally provided in the form of beads or granules, which by effect of the heat and in particular of the water vapor, are firstly expanded till the desired density is reached and, after maturation in the air, are molded in shape to obtain expanded blocks and shaped articles.
The obtained expanded articles have a closed cell structure and are suitable to be used in important fields, such as in the building, refrigeration, packaging fields, etc.
These expandable beads are obtained by means of a polymerization process in suspension, appropriately combined with an impregnation procedure of the polymer with the expanding agent.
This process, that is of discontinuous kind, provides a product of good quality, but of a rather dispersed granulometry. That involves a subsequent screening to remove out of size particles.
The polymerization process in suspension involves moreover substantial restrictions concerning the production of particular products, such as for instance the production of colored expandable beads, because some kinds of dyes or pigments either can be incorporated into the polymer with difficulty or alternatively they can inhibit the polymerization. The same drawback can happen when nucleating agents are concerned, which are generally added to improve the cellular structure of the expanded polymer.
Another process, that is industrially less used, for the production of expandable polystyrene, consists in adding the expanding agent into the molten polymer during the extrusion, in drawing the resulting mass to spaghetti, in cooling and in cutting them at a distance to expandable granules.
This type of process foresees the feed of polystyrene granules into an extruder with continuous injection, by means of a metering pump, of an expanding agent into the extrusion melt.
The resulting expandable mass is drawn to spaghetti, which are cooled in cold water in order to bring them rapidly from the extrusion temperature to a temperature lower than the Tg of the expandable polymer.
This rapid cooling allows to avoid the polymer expansion, but it presents the great drawback to induce into the granules orientations and stresses caused by quenching, which, as known, affects negatively the uniformity of the cellular structure and the performances of the expanded product.
In order to improve the quality of the expandable granules obtained by extrusion, it was proposed to eliminate the orientations and the stresses frozen in them, by means of a subsequent annealing at a temperature lower than the Tg of the polymer at room pressure.
Obviously this operation needs an additional stage, that requires relatively long times. Furthermore since it has to be carried out at a temperature lower than the Tg of the expandable polymer to avoid the granule expansion, this annealing operation does not allow to eliminate completely the above mentioned drawbacks.
In order to eliminate them, on the contrary, it should be necessary to work at a temperature higher than the Tg of the polymer and to cool slowly. This process, however, involves the risk of the non-desired expansion of the material.